Vitamins are denoted by letters and are often divided into fat-soluble (A, D, E, and K) and water-soluble (B and C).
Vitamins are needed only by heterotrophic organisms; autotrophs, such as most green plants, are by definition independent of an external supply of organic compounds. What is a vitamin for one heterotroph may be synthesized in adequate amounts by, and is therefore not a vitamin for, another; or it may take no part at all in the metabolism of another. There is no such thing as a vitamin in general, but only for specified organisms. Some (like those of the vitamin B complex) which are perhaps universal constituents of existing organisms are however required by a very wide range of organism; others (like C) by very few. Every vitamin necessary for a given organism is synthesized by other organisms, otherwise a continuous supply wouldn't be available.
Sometimes several different compounds can substitute for each other in satisfying a given requirement; either because the organism requires, not a specific molecule, but a specific chemical grouping which is present in, and available from, each of the alternative compounds; or because conversion of a few closely related groupings into the one required is possible within the organism. Deficiency of the vitamin reduces the rate of the metabolic process in which it is concerned, with widespread effects (symptoms of deficiency disease). A general effect of deficiency of most vitamins, which was important in the early history of their discovery, is that growth of young animals is stunted.
Function of specific vitamins and effect of deficiency
Important members of the vitamin B group include thiamine (B1), riboflavin (B2), niacin, pyridoxine (B6), folic acid, and cyanobalamin B12).
Vitamin C, or ascorbic acid, is involved in many metabolic pathways and has an important role in healing, blood cell formation, and bone and tissue growth; scurvy is its deficiency disease.
Vitamin D, or calciferol, is a crucial factor in calcium metabolism, including the growth and structural maintenance of bone; lack causes rickets, while overdosage also causes disease.
Vitamin E, or tocopherol, appears to play a role in blood cell and nervous system tissues, but its deficiency is uncommon and its beneficial properties have probably been overstated.
Vitamin K provides essential cofactors for production of certain clotting factors in the liver; it is used to treat some clotting disorders, include that seen in some premature infants.
Sources of vitaminsDietary sources of vitamin are shown in the illustration to the right. Vitamins essential to health are found in great quantity in liver, milk, green vegetables, legumes, and corn. Fatty foods provide the chief source of vitamins K, D, A, and E, which are soluble in fats and oils. Vitamin K is produced by micro-organisms in the gut, while any ordinary diet contains adequate vitamin E. Some vitamin D is produced in the skin on exposure to sunlight but this is insufficient except in sunny climates. Though they are essential nutrients, vitamins A and D are poisonous in large doses. Vitamin B is generally found in meat and bran cereals. Vitamin C, found in fresh vegetables and fruit, is easily lost in cooking and storage.
Related category• BIOCHEMISTRY
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